This invention relates to printers and particularly printers having a print head platen for guiding the print medium.
Typically ink-jet printers, or any printers using wet ink, include a printhead, a print zone positioned adjacent the printhead, a feed mechanism for feeding a print medium through the print zone, and a platen positioned adjacent the print zone, the platen guiding and supporting the print medium in the print zone during printing.
During printing, ink is placed on the print medium by dropping or ejecting ink from the printhead, or by any other printing method well known by those skilled in the art. Ink used in wet ink-type printing includes a relatively large amount of water. As the wet ink contacts the print medium, the water in the ink saturates the fibers of the print medium, causing the fibers to expand, which in turn causes the print medium to buckle.
Buckling, also called cockling, of the print medium tends to cause the print medium either to uncontrollably bend downwardly away from the printhead, or to uncontrollably bend upwardly toward the printhead. In either case, a constant pen-to-print medium spacing is not achieved, leading to poor print quality. Additionally, upwardly buckling print medium may contact a pen nozzle in the printhead, leading to ink smearing on the print medium.
Typically, to achieve good print quality, pen-to-print medium spacing of less than 1.5 millimeters (mm), and preferably less than 1.0 mm, is required. However, bending amplitudes of print medium in certain pen/ink combinations can be greater than 3 mm. To reduce this problem of paper buckling, which varies the pen-to-print medium spacing, various platen designs have been proposed.
One approach is to use a flat platen, with the platen supporting the print medium throughout a print zone defined between the printhead and the platen.
Another approach is to use a platen having raised ribs. These raised ribs support the print medium with a small spacing above the main body of the platen. This spacing improves the drying of the deposited ink and reduces friction between the print medium and the platen, thereby improving the drive of the print medium through the device.
It is also known to apply a vacuum pressure to the space between the print medium and the platen base. This can be achieved by providing holes in the platen, and drawing air in through the holes, for example using a fan.
The different features of the platen, such as rib size and spacing, need to be selected depending on the print medium material and the dimensions of the print head, as these factors all influence the friction levels encountered, which in turn dictate the design of the feed mechanism.